1. Field of the Invention
The present invention concerns an electronic variable speed drive including, in a casing, a rectifier supplied with alternating current and associated with capacitors to provide a direct current supply to an inverter comprising a plurality of solid state switch power modules controlled by an electronic control system and supplying an AC voltage.
1. Discussion of the Background
Electronic variable speed drives are routinely used to drive direct current motors and alternating current motors at variable speed.
Frequency converter type variable speed drives derive from the single-phase or three-phase AC mains an AC voltage with a root means square value and frequency that vary according to a predefined law in order to control the speed of alternating current motors. These frequency converters essentially comprise a rectifier associated with a smoothing capacitor filter to supply a filtered DC voltage to an inverter. The rectifier is a rectifier bridge connected to the single-phase or three-phase AC mains. The inverter supplies a variable frequency AC voltage at its output. The inverter includes solid-state switches called power modules and is connected to the phase windings of the motor. It produces an AC voltage from the DC voltage supplied by the rectifier.
The solid-state power switches are controlled by control electronics. One well-known control technique is pulse width modulation (PWM). Its objective is to generate a voltage as close as possible to a sinusoid.
For optimum ventilation of the variable speed drive the casing incorporates a plurality of openings. In the event of a short-circuit, flames around the power modules cause short-circuits to the earthed metal parts.